The invention relates generally to electrical machines and more particularly, to magnetization of permanent magnet rotors in electrical machines.
Generally, an electrical machine, such as a motor or a generator, includes a rotor disposed within a stator and utilized to convert electrical power to mechanical power or vice versa. Certain electrical machines use permanent magnet type rotors, which rotors reduce the size and enhance the overall efficiency of the machine. Such a rotor generally includes an annular permanent magnet, disposed over a rotor spindle. In larger machines, the permanent magnet is generally formed by assembling a plurality of permanent magnets assembled around a rotor spindle. In general, the permanent magnet segments are magnetized prior to assembly on the rotor spindle. For example, the permanent magnet segments are cut and ground to shape from larger unfinished magnet blocks, after which the segments are magnetized individually in a solenoid coil. In certain applications, especially in larger machines, magnetization of the permanent magnet segments is achieved via a magnetization vector proposed by K. Halbach (also known as Halbach magnetization), which, when applied to the surface of the permanent magnets, results in a more sinusoidal shaped flux distribution within the electrical machine, thereby reducing AC harmonic losses and reducing torque ripple, vibration and acoustic noise. The permanent magnet segments are subsequently adhesively bonded to the rotor spindle.
However, assembly of the rotor from pre-magnetized permanent magnet segments may be a cumbersome process, especially in larger electrical machines, as it may be time consuming and unwieldly. The process can involve substantial forcing and aligning by mechanical devices to position and restrain the energized permanent magnet segments. The process is prone to physical accidents if energized permanent magnet blocks escape restraint. Thus, for farbrication convenience, the rotor is assembled with un-magnetized magnets and magnetized all at once by imparting necessary magnetic fields to the un-magnetized magnets. Further, electrical machines such as a high-speed electrical motor may also include a holding ring or a retaining ring around the permanent magnet assembly on the rotor to prevent fracturing and scattering of the permanent magnet assembly by centrifugal forces. Often the retaining ring is made of a metallic material, thereby, the retaining ring inherits metallic property of having electrical conductivity to a substantial degree. During magnetization of the un-magnetized magnets, the metallic retaining ring or holding ring around the permanent magnet assembly is induced with eddy currents which impede the un-magnetized magnets from becoming magnetized.
Accordingly, there exists a need for a simpler and efficient technique for magnetization of the un-magnetized magnet segments in electrical machine rotors, wherein a metallic retaining ring is utilized for magnet retention in a permanent magnet AC synchronous machine.